Have you ever looked over your spouse’s shoulder and thought “Hey, that’s a cool app.”? If you set up Family Sharing (in System Preferences > iCloud on the Mac and in Settings > Your Name in iOS), you can download almost any app that someone else in your family has purchased on either the Mac App Store or the iOS App Store.
How you find these shared apps depends on the platform. On a Mac running macOS 10.14 Mojave; open the App Store app, click your email address at the bottom of the sidebar and then click the name next to “Purchased by” to see another family member’s purchases.
In iOS 12’s App Store app; tap your icon at the upper right, tap Purchased and then tap a family member to see their purchases (note that you can select Not on this iPhone/iPad to narrow the choices). Click or tap the cloud icon to download a purchased app.